The present invention relates to a process for the gas phase selective orthomethylation of phenols having at least one hydrogen atom in the ortho position.
It is known from DAS No. 2,127,083 (=British Pat. No. 1,323,211) that methanol can be reacted with phenols of the mentioned general structure on a catalyst containing manganese oxide and cerium oxide at temperatures of 300.degree.-550.degree. C. British Pat. No. 717,588 discloses that methanol reacts with o-cresol in the gaseous phase in the presence of a catalyst made up of only one of the following metallic oxides: oxides of magnesium, aluminum, calcium, manganese, iron, zinc, zirconium, barium, or thorium. British Pat. No. 1,124,839 describes a process for the production of alkyl phenols wherein a phenol is reacted with an alcohol in the presence of an oxide of a rare earth element, such as, for example, cerium, lanthanum, neodymium, and praseodymium. All of the catalysts utilized in these processes contain only one of the active components, iron oxide or cerium oxide.
These conventional catalysts exhibit several disadvantages. They have low activity and low selectivity for the orthoalkylation. Additionally, they result in a high loss of methanol. Also, the lifetime of these catalysts is short.
Furthermore, catalysts comprised of iron oxide and silicic acid (hydrated silica) or of iron oxide, silicic acid, and chromium oxide have been suggested for the preparation or orthoalkyl phenols with high selectivity (DAS No. 2,428,056=British Pat. No. 1,428,057). However, the binary catalyst also exhibits a strong reduction in activity during a prolonged operating period. The ternary catalyst, on the other hand, does have a longer lifetime compared with the binary catalyst. Moreover, this can be still further improved by using a quaternary catalyst consisting of iron oxide, silicic acid, chromium oxide, and an alkali metal compound or alkaline earth metal compound (German Pat. No. 2,547,309=British Pat. No. 1,507,478 and Belgian Pat. No. 888,022=British Pat. No. 2,072,674). However, such lifetimes are still inadequate for industrial utilization.